The stability with respect to a peeling-ballooning mode (PBM) was investigated numerically with extended MHD simulation codes in JET, JT-60U and future JT-60SA plasmas. The MINERVA-DI code was used to analyze the linear stability, including the effects of rotation and ion diamagnetic drift (omega(*i)), in JET-ILW and JT-60SA plasmas, and the JOREK code was used to simulate nonlinear dynamics with rotation, viscosity and resistivity in JT-60U plasmas. It was validated quantitatively that the ELM trigger condition in JET-ILW plasmas can be reasonably explained by taking into account both the rotation and omega(*i) effects in the numerical analysis. When deuterium poloidal rotation is evaluated based on neoclassical theory, an increase in the effective charge of plasma destabilizes the PBM because of an acceleration of rotation and a decrease in omega(*i). The difference in the amount of ELM energy loss in JT-60U plasmas rotating in opposite directions was reproduced qualitatively with JOREK. By comparing the ELM affected areas with linear eigenfunctions, it was confirmed that the difference in the linear stability property, due not to the rotation direction but to the plasma density profile, is thought to be responsible for changing the ELM energy loss just after the ELM crash. A predictive study to determine the pedestal profiles in JT-60SA was performed by updating the EPED1 model to include the rotation and w*i effects in the PBM stability analysis. It was shown that the plasma rotation predicted with the neoclassical toroidal viscosity degrades the pedestal performance by about 10% by destabilizing the PBM, but the pressure pedestal height will be high enough to achieve the target parameters required for the ITER-like shape inductive scenario in JT-60SA.

Electron temperature profiles have been measured by the main Thomson scattering ( TS) diagnostic on the RFX-mod reversed field pinch experiment in Padova, Italy. The increased accuracy and spatial and temporal resolution permits one to measure in detail the improvements in T-e profiles, obtained with the active saddle coil system, which allows one to obtain core temperature 30% higher and scaling stronger with plasma current, steeper gradients in the core (+30%) and at the edge (+60%). 1D power balance calculations show that the active control of MHD modes largely reduces the values of electron heat diffusivity along the whole plasma radius, with similar to 50% reduction at the edge and similar to 30% in the core. The resulting electron energy confinement time is doubled. Further improvements occur during quasi-single helicity (QSH) states: the new TS allows one to study in detail the hot island that develops in the core. A characterization of the island electron thermal profile is presented, in terms of width, temperature increase, gradients and asymmetry; the effect on density profile is also discussed. A 2D transport code has been applied to calculate the heat diffusivity inside the magnetic island corresponding to the QSH state, also considering the correlation between temperature increase and pressure gradient with the chaos level around the island. Finally, electron energy confinement time during QSH states is compared with that in MH states.

Observation of magnetic activity in several discharges in the Frascati Tokamak Upgrade has revealed high-frequency oscillations that accompany the development of large magnetic islands in ohmic plasmas. The frequency of these oscillations is one order of magnitude above the typical island rotation frequency and one order of magnitude below the toroidicity-induced gap of the shear-Alfvén continuum. By writing a precise dispersion relation, we interpret these modes as beta-induced Alfvén eigenmodes, i.e. Alfvén eigenmodes located in the low frequency gap which is caused by finite compressibility.

Starting from recent JET experimental results that show a significant reduction of ion stiffness in the plasma core region due to plasma rotation in the presence of low magnetic shear, an experiment was carried out at JET in order to separate the role of rotation and rotation gradient in mitigating the ion stiffness level. Enhanced toroidal field ripple (up to 1.5%) and external resonant magnetic fields are the two mechanisms used to try and decouple the rotation value from its gradient. In addition, shots with reversed toroidal field and plasma current, yielding counter-current neutral beam injection, were compared with standard co-injection cases. These tools also allowed varying the rotation independently of the injected power. Shots with high rotation gradient are found to maintain their low stiffness level even when the absolute value of the rotation was significantly reduced. Conversely, high but flat rotation yields much less peaked ion temperature profiles than a peaked rotation profile with lower values. This behaviour suggests the rotation gradient as the main player in reducing the ion stiffness level. In addition, it is found that inverting the rotation gradient sign does not suppress its effect on ion stiffness.

The dependence of the H-mode edge transport barrier width on normalized ion gyroradius (rho* = rho/a) in discharges with type I ELMs was examined in experiments combining data for the JET and DIII-D tokamaks. The plasma configuration as well as the local normalized pressure (beta), collisionality (nu*), Mach number and the ratio of ion and electron temperature at the pedestal top were kept constant, while rho* was varied by a factor of four. The width of the steep gradient region of the electron temperature (T-e) and density (n(e)) pedestals normalized to machine size showed no or only a weak trend with rho*. A rho(1/2) or rho(1) dependence of the pedestal width, given by some theoretical predictions, is not supported by the current experiments. This is encouraging for the pedestal scaling towards ITER as it operates at lower rho* than existing devices. Some differences in pedestal structure and ELM behaviour were, however, found between the devices; in the DIII-D discharges, the n(e) and T-e pedestal were aligned at high rho* but the ne pedestal shifted outwards in radius relative to T-e as rho* decreases, while on JET the profiles remained aligned while rho* was scanned by a factor of two. The energy loss at an ELM normalized to the pedestal energy increased from 10% to 40% as rho* increased by a factor of two in the DIII-D discharges but no such variation was observed in the case of JET. The measured pedestal pressures and widths were found to be consistent with the predictions from modelling based on peeling-ballooning stability theory, and are used to make projections towards ITER

In both JET and ASDEX Upgrade (AUG) the plasma energy confinement has been affected by the presence of a metal wall by the requirement of increased gas fuelling to avoid tungsten pollution of the plasma. In JET with a beryllium/tungsten wall the high triangularity baseline H-mode scenario (i.e. similar to the ITER reference scenario) has been the strongest affected and the benefit of high shaping to give good normalized confinement of H-98 similar to 1 at high Greenwald density fraction of f(GW) similar to 0.8 has not been recovered to date. In AUG with a full tungsten wall, a good normalized confinement H-98 similar to 1 could be achieved in the high triangularity baseline plasmas, albeit at elevated normalized pressure beta(N) > 2. The confinement lost with respect to the carbon devices can be largely recovered by the seeding of nitrogen in both JET and AUG. This suggests that the absence of carbon in JET and AUG with a metal wall may have affected the achievable confinement. Three mechanisms have been tested that could explain the effect of carbon or nitrogen (and the absence thereof) on the plasma confinement. First it has been seen in experiments and by means of nonlinear gyrokinetic simulations (with the GENE code), that nitrogen seeding does not significantly change the core temperature profile peaking and does not affect the critical ion temperature gradient. Secondly, the dilution of the edge ion density by the injection of nitrogen is not sufficient to explain the plasma temperature and pressure rise. For this latter mechanism to explain the confinement improvement with nitrogen seeding, strongly hollow Z(eff) profiles would be required which is not supported by experimental observations. The confinement improvement with nitrogen seeding cannot be explained with these two mechanisms. Thirdly, detailed pedestal structure analysis in JET high triangularity baseline plasmas have shown that the fuelling of either deuterium or nitrogen widens the pressure pedestal. However, in JET-ILW this only leads to a confinement benefit in the case of nitrogen seeding where, as the pedestal widens, the obtained pedestal pressure gradient is conserved. In the case of deuterium fuelling in JET-ILW the pressure gradient is strongly degraded in the fuelling scan leading to no net confinement gain due to the pedestal widening. The pedestal code EPED correctly predicts the pedestal pressure of the unseeded plasmas in JET-ILW within +/- 5%, however it does not capture the complex variation of pedestal width and gradient with fuelling and impurity seeding. Also it does not predict the observed increase of pedestal pressure by nitrogen seeding in JET-ILW. Ideal peeling ballooning MHD stability analysis shows that the widening of the pedestal leads to a down shift of the marginal stability boundary by only 10-20%. However, the variations in the pressure gradient observed in the JET-ILW fuelling experiment is much larger and spans a factor of more than two. As a result the experimental points move from deeply unstable to deeply stable on the stability diagram in a deuterium fuelling scan. In AUG-W nitrogen seeded plasmas, a widening of the pedestal has also been observed, consistent with the JET observations. The absence of carbon can thus affect the pedestal structure, and mainly the achieved pedestal gradient, which can be recovered by seeding nitrogen. The underlying physics mechanism is still under investigation and requires further understanding of the role of impurities on the pedestal stability and pedestal structure formation.

Infrequent, bursty, electromagnetic, whistler-mode wave packets, excited spontaneously in the laboratory by an electron beam from a hot cathode, appear transiently, each with a time duration tau around similar to 1 mu s. The wave packets have a center frequency f(W) that is broadly distributed in the range 7 MHz < f(W) < 40 MHz. They are excited in a region with separate electrostatic (es) plasma oscillations at values of f(hf), 200 MHz < f(hf) < 500 MHz, that are hypothesized to match eigenmode frequencies of an axially localized hf es field in a well-defined region attached to the cathode. Features of these es-eigenmodes that are studied include: the mode competition at times of transitions from one dominating es-eigenmode to another, the amplitude and spectral distribution of simultaneously occurring es-eigenmodes that do not lead to a transition, and the correlation of these features with the excitation of whistler mode waves. It is concluded that transient coupling of es-eigenmode pairs at f(hf) such that vertical bar f(1, hf) - f(2, hf)vertical bar = f(W) < f(ge) can explain both the transient lifetime and the frequency spectra of the whistler-mode wave packets (f(W)) as observed in lab. The generalization of the results to bursty whistler-mode excitation in space from electron beams, created on the high potential side of double layers, is discussed.

The EXTRAP T2R thin shell reversed-field pinch (RFP) device has recently resumed operation after a major rebuild including the replacement of the graphite armour with molybdenum limiters, a fourfold increase of the shell time constant, and the replacement of the helical coil used for the toroidal field with a conventional solenoid-type coil. Wall-conditioning using hydrogen glow discharge cleaning was instrumental for successful RFP operation. Carbon was permanently removed from the walls during the first week of operation. The initial results from RFP operation with relatively low plasma currents in the range I-p = 70-100 kA are reported. RFP discharges are sustained for more than three shell times. Significant improvements in plasma parameters are observed, compared to operation before the rebuild. There is a substantial reduction in the carbon impurity level. The electron density behaviour is more shot-to-shot reproducible. The typical density is n(e) = 0.5-1 x 10(19) m(-3). Monitors of H-alpha line radiation indicate that the plasma wall interaction is more toroidally symmetric and that there is less transient gas release from the wall. The minimum loop voltage is in the range V-t = 28-35 V, corresponding to a reduction by a factor of two to three compared to the value before the rebuild.

A two-dimensional array of saddle coils at M-c poloidal and N-c toroidal positions is used on the EXTRAP T2R reversed-field pinch (Brunsell P R et al 2001 Plasma Phys. Control. Fusion 43 1457) to study active control of resistive wall modes (RWMs). Spontaneous growth of several RWMs with poloidal mode number m = 1 and different toroidal mode number n is observed experimentally, in agreement with linear MHD modelling. The measured plasma response to a controlled coil field and the plasma response computed using the linear circular cylinder MHD model are in quantitive agreement. Feedback control introduces a linear coupling of modes with toroidal mode numbers n, n' that fulfil the condition vertical bar n - n'vertical bar = N-c. Pairs of coupled unstable RWMs are present in feedback experiments with an array of Mc x Nc = 4 x 16 coils. Using intelligent shell feedback, the coupled modes are generally not controlled even though the field is suppressed at the active coils. A better suppression of coupled modes may be achieved in the case of rotating modes by using the mode control feedback scheme with individually set complex gains. In feedback with a larger array of Mc x Nc = 4 x 32 coils, the coupling effect largely disappears, and with this array, the main internal RWMs n = -11, -10, +5, +6 are all simultaneously suppressed throughout the discharge (7-8 wall times). With feedback there is a two-fold extension of the pulse length, compared to discharges without feedback.

Experimental data on dust resident in the EXTRAP T2R reversed-field pinch are reported. Mobile dust grains are captured in situ by silicon collectors, whereas immobile grains are sampled post mortem from the wall by adhesive tape. The simulation of collection asymmetries by the MIGRAINe dust dynamics code in combination with the experimental results is employed to deduce some characteristics of the mechanism of intrinsic dust release. All evidence suggests that re-mobilization is dominant with respect to dust production.

The feasibility of the optical and electrical detection of dust-impact ionization events in the scrape-off layers of tokamak plasmas is evaluated. It is shown that the expected light emission and the charge released during a dust impact on a biased target can be measured above the light emission and the charge collected due to the background plasma. A scheme of an electro-optical probe for diagnostics of fast dust particles is proposed.

Pulsed poloidal current drive (PPCD) experiments have been conducted in the resistive shell EXTRAP T2R reversed-field pinch experiment. During the current profile modification phase, the fluctuation level of the m = 1 internally resonant tearing modes decreases, and the velocity of these modes increases. The m = 0 modes are not affected during PPCD, although termination occurs with a burst in the m = 0 amplitude. The PPCD phase is characterized by an increase in the central electron temperature (up to 380 eV) and in the soft x-ray signal. Spectroscopic observations confirm an increase in the central electron temperature. During PPCD, the plasma poloidal beta increases to 14%, and the estimated energy confinement time doubles, reaching 380 mus. The reduction in the fluctuation level and the corresponding increase in the energy confinement time are qualitatively consistent with a reduction in parallel transport along stochastic magnetic field lines.

An extensive investigation of the global confinement properties in different operating scenarios in the rebuilt EXTRAP T2R reversed-field pinch (RFP) experiment is reported here. In particular, the role of a fast gas puff valve system, used to control plasma density, on confinement is studied. Without gas puffing, the electron density decays below 0.5 x 10(19) M-3. The poloidal beta varies between 5% and 15%, decreasing at large I/N. The energy confinement time ranges from 70 to 225 mus. With gas puffing, the density is sustained at n(e) approximate to 1.5 x 10(19) m(-3). However, a general slight deterioration of the plasma performances is observed for the same values of I/N: the plasma becomes cooler and more radiative. The poloidal beta is comparable to that in the scenarios without puff but the energy confinement time drops ranging from 60 to 130 mus. The fluctuation level and the energy confinement time have been found to scale with the Lundquist number as S-0.05+/-0.07 and S0.5+/-0.1, respectively. Mode rotation is typical for all the discharges and rotation velocity is observed to increase with increasing electron diamagnetic velocity.

Active feedback stabilization of multiple resistive wall modes (RWMs) has been successfully proven in the EXTRAP T2R reversed field pinch. One of the features of plasma discharges operated with active feedback stabilization, in addition to the prolongation of the plasma discharge, is the sustainment of the plasma rotation. Sustained rotation is observed both for the internally resonant tearing modes (TMs) and the intrinsic impurity oxygen ions. Good quantitative agreement between the toroidal rotation velocities of both is found: the toroidal rotation is characterized by an acceleration phase followed, after one wall time, by a deceleration phase that is slower than in standard discharges. The TMs and the impurity ions rotate in the same poloidal direction with also similar velocities. Poloidal and toroidal velocities have comparable amplitudes and a simple model of their radial profile reproduces the main features of the helical angular phase velocity. RWMs feedback does not qualitatively change the TMs behaviour and typical phenomena such as the dynamo and the `slinky' are still observed. The improved sustainment of the plasma and TMs rotation occurs also when feedback only acts on internally non- resonant RWMs. This may be due to an indirect positive effect, through non- linear coupling between TMs and RWMs, of feedback on the TMs or to a reduced plasma- wall interaction affecting the plasma flow rotation. Electromagnetic torque calculations show that with active feedback stabilization the TMs amplitude remains well below the locking threshold condition for a thick shell. Finally, it is suggested that active feedback stabilization of RWMs and current profile control techniques can be employed simultaneously thus improving both the plasma duration and its confinement properties.

Numerical modelling of the effects of ion cyclotron resonance heating (ICRH) on the stability of the internal kink mode suggests that ICRH should be considered as an essential sawtooth control tool in ITER. Sawtooth control using ICRH is achieved by directly affecting the energy of the internal kink mode rather than through modification of the magnetic shear by driving localized currents. Consequently, ICRH can be seen as complementary to the planned electron cyclotron current drive actuator, and indeed will improve the efficacy of current drive schemes. Simulations of the ICRH distribution using independent RF codes give confidence in numerical predictions that the stabilizing influence of the fusion-born alphas can be negated by appropriately tailored minority (3)He ICRH heating in ITER. Finally, the effectiveness of all sawtooth actuators is shown to increase as the q = 1 surface moves towards the manetic axis, whilst the passive stabilization arising from the alpha and NBI particles decreases.

Long period sawteeth have been observed to result in low-beta triggering of neo-classical tearing modes, which can significantly degrade plasma confinement. Consequently, a detailed physical understanding of sawtooth behaviour is critical, especially for ITER where fusion-born a particles are likely to lead to very long sawtooth periods. Many techniques have been developed to control, and in particular to destabilize the sawteeth. The application of counter-current neutral beam injection (NBI) in JET has resulted in shorter sawtooth periods than in Ohmic plasmas. This result has been explained because, firstly, the counter-passing fast ions give a destabilizing contribution to the n=1 internal kink mode-which is accepted to be related to sawtooth oscillations-and secondly, the flow shear strongly influences the stabilizing trapped particles. A similar experimental result has been observed in counter-NBI heated plasmas in MAST. However, the strong toroidal flows in spherical tokamaks mean that the sawtooth behaviour is determined by the gyroscopic flow stabilization of the kink mode rather than kinetic effects. In NBI heated plasmas in smaller conventional aspect-ratio tokamaks, such as TEXTOR, the flow and kinetic effects compete to give different sawtooth behaviour. Other techniques applied to destabilize sawteeth are the application of electron cyclotron current drive (ECCD) or ion cyclotron resonance heating (ICRH). In JET, it has been observed that localized ICRH is able to destabilize sawteeth which were otherwise stabilized by a co-existing population of energetic trapped ions in the core. This is explained through the dual role of the ICRH in reducing the critical magnetic shear required to trigger a sawtooth crash, and the increase in the local magnetic shear which results from driving current near the q=1 rational surface. Sawtooth control in ITER could be provided by a combination of ECCD and co-passing off-axis negative-NBI fast ions.

The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-beta JET L-mode discharge illustrates the relation between ITG stabilization and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-beta JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-beta generalization of comparable observations found at low-beta at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

The performance of the 'hybrid' H-mode regime (long pulse operation with high neutron fluency) has been extensively investigated in JET during the 2005-2007 experimental campaign up to normalized pressure beta(N) = 3, toroidal magnetic field B-t = 1.7T, with type I ELMs plasma edge conditions. The optimized external current drive sources, self-generated non-inductive bootstrap current and plasma core stability properties provide a good prospect of achieving a high fusion gain at reduced plasma current for long durations in ITER. One of the remaining issues is the erosion of the divertor target plates associated with the type I ELM regime. A possible solution could be to operate with a plasma edge in the type III ELM regime (reduced transient and stationary heat loads) obtained with impurity seeding. An integrated hybrid type III ELM regime with a normalized pressure beta(N) = 2.6 (PNBI similar to 20-22 MW) and a thermal confinement factor of H-98* 98(y, 2) similar to 0.83 has been recently successfully developed on JET with nitrogen seeding. This scenario shows good plasma edge condition (compatible with the future ITER-like wall on JET) and moderate MHD activity. In this paper, we report on the experimental development of the scenario (with plasma current I-p = 1.7MA and magnetic field B-t = 1.7T) and the trade-off between heat load reduction at the target plates and global confinement due to nitrogen seeding and type III ELM working conditions.

Mechanisms underlying the retention of fuel species in tokamaks with carbon plasma-facing components are presented, together with estimates for the corresponding retention of tritium in ITER. The consequential requirement for new and improved schemes to reduce the tritium inventory is highlighted and the results of ongoing studies into a range of techniques are presented, together with estimates of the tritium removal rate in ITER in each case. Finally, an approach involving the integration of many tritium removal techniques into the ITER operational schedule is proposed as a means to extend the period of operations before major intervention is required.

Quenching of the reversed-field pinch (RFP) dynamo is observed in numerical simulations using current profile control. A novel algorithm employing active feedback of the dynamo field has been utilized. The quasi-steady state achieved represents an important improvement as compared with earlier numerical work and may indicate a direction for the design of future experiments. Both earlier and the novel schemes of feedback control result in quasi-single helicity states. The energy confinement time and poloidal beta are observed to be substantially increased, as compared with the conventional RFP, in both the cases. Different techniques for experimental implementation are discussed.

The effect of a toroidal field (TF) ripple on the formation and performance of internal transport barriers (ITBs) has been studied in JET. It was found that the TF ripple had a profound effect on the toroidal plasma rotation. An increased TF ripple up to delta = 1% led to a lower rotation and reduced the rotational shear in the region where the ITBs were formed. ITB triggering events were observed in all cases and it is thought that the rotational shear may be less important for this process than, for example, the q-profile. However, the increase in the pressure gradient following the ITB trigger was reduced in discharges with a larger TF ripple and consequently a lower rotational shear. This suggests that toroidal rotation and its shear play a role in the growth of the ITB once it has been triggered.

The electric field is a quantity of particular relevance in plasma physics. Indeed, its fluctuations are responsible for different macroscopic phenomena such as anomalous transport in fusion plasmas. Answering a long-standing challenge, we offer a new method to locally and non-intrusively measure weak electric fields and their fluctuations in plasmas, by means of a beam of hydrogen ions or atoms. We present measurements of the electric field in vacuum and in a plasma where Debye shielding is measured. For the first time, we have used the Lamb-shift resonance to measure oscillating electric fields around 1 GHz and observed the strong enhancement of the Lyman-alpha signal. The measurement is both direct and non-intrusive. This method provides sensitivity (mV cm(-1)) and temporal resolution (ns) that are three orders higher compared to current diagnostics. It thus allows measuring fluctuations of the electric field at scales not previously reached experimentally.

Results of experimental scans of heating power, plasma shape, and nitrogen content are presented, with a focus on global performance and pedestal alteration. In detailed scans at low triangularity, it is shown that the increase in stored energy due to nitrogen seeding stems from the pedestal. It is also shown that the confinement increase is driven through the temperature pedestal at the three heating power levels studied. In a triangularity scan, an orthogonal effect of shaping and seeding is observed, where increased plasma triangularity increases the pedestal density, while impurity seeding (carbon and nitrogen) increases the pedestal temperature in addition to this effect. Modelling of these effects was also undertaken, with interpretive and predictive models being employed. The interpretive analysis shows a general agreement of the experimental pedestals in separate power, shaping, and seeding scans with peeling-ballooning theory. Predictive analysis was used to isolate the individual effects, showing that the trends of additional heating power and increased triangularity can be recoverd. However, a simple change of the effective charge in the plasma cannot explain the observed levels of confinement improvement in the present models.

Experimental evidence for the impact of a region of high density localised in the high-field side scrape-off layer (the HFSHD) on plasma confinement is shown in various dedicated experiments on ASDEX Upgrade (AUG). Increasing main ion fuelling is shown to increase the separatrix density and shift the density profile outwards. Predictive pedestal modelling of this shift indicates a 25% decrease in the attainable pedestal top pressure, which compares well with experimental observations in the gas scan. Since the HFSHD can be mitigated by applying nitrogen seeding, a combined scan in fuelling rate, heating power, and nitrogen seeding is presented. Significant increases in the achievable pedestal top pressure are observed with seeding, in particular at high heating powers, and are correlated with inward shifted density profiles and a reduction of the HFSHD and separatrix density. Interpretive linear stability analysis also confirms the impact of a radially shifted pressure profile on peeling-ballooning stability, with an inward shift allowing access to higher pressure gradients and pedestal widths.

Reliable coupling of the lower hybrid current drive (LHCD) to H-mode plasmas in JET is made feasible through a dedicated gas injection system, located at the outer wall and magnetically connected to the antenna (Pericoli Ridolfini et al 2004 Plasma Phys. Control. Fusion 46 349, Ekedahl et al 2005 Nucl. Fusion 45 351, Ekedahl et al 2009 Plasma Phys. Control. Fusion 51 044001). An experiment was carried out in JET in order to investigate whether a gas injection from the top of the torus, as is foreseen for the main gas injection in ITER, could also provide good coupling of the LH waves if magnetically connected to the antenna. The results show that a top gas injection was not efficient for providing a reliable LHCD power injection, in spite of being magnetically connected and in spite of using almost twice the amount of gas flow compared with the dedicated outer mid-plane gas puffing system. A dedicated gas injection system, set in the outer wall and magnetically connected to the LHCD antenna, is therefore recommended in order to provide the reliable coupling conditions for an LHCD antenna in ITER.

Good coupling of lower hybrid (LH) waves has been demonstrated in different H-mode scenarios in JET, at high triangularity (delta similar to 0.4) and at large distance between the last closed flux surface and the LH launcher ( up to 15 cm). Local gas injection of D-2 in the region magnetically connected to the LH launcher is used for increasing the local density in the scrape-off layer ( SOL). Reciprocating Langmuir probe measurements magnetically connected to the LH launcher indicate that the electron density profile flattens in the far SOL during gas injection and LH power application. Some degradation in normalized H-mode confinement, as given by the H98(gamma,2)-factor, could be observed at high gas injection rates in these scenarios, but this was rather due to total gas injection and not specifically to the local gas puffing used for LH coupling. Furthermore, experiments carried out in L-mode plasmas in order to evaluate the effect on the LH current drive efficiency, when using local gas injection to improve the coupling, indicate only a small degradation (Delta I-LH/I-LH similar to 15%). This effect is largely compensated by the improvement in coupling and thus increase in coupled power when using gas puffing.

A model for treating ripple induced fast ion losses during ion cyclotron resonance frequency heating is presented. It is suitable for codes solving an orbit averaged three-dimensional Fokker-Planck equation with a Monte Carlo method, and has been implemented in such a code. The resulting code has been used for a comparison with experimental data form Tore Supra and for assessing the ripple induced losses in different ICRF heating scenarios.

Observations of bulk plasma rotation in radio frequency (RF) heated JET discharges are reported. This study is concentrated on RF heated L-mode plasmas. In particular, the toroidal rotation profiles in plasmas heated by ion cyclotron resonance frequency (ICRF) waves and lower hybrid (LH) waves have been analysed. It is the first time that rotation profiles in JET plasmas with LH waves have been measured in dedicated discharges. It is found that the toroidal plasma rotation in the outer region of the plasmas is in the co-current direction irrespective of the heating scenario. An interesting feature is that the toroidal rotation profile appears to be hollow in many discharges at low plasma current, but a low current in itself does not seem to be a sufficient condition for finding such profiles. Fast ion transport and finite orbit width effects are mechanisms that could explain hollow rotation profiles. This possibility has been investigated by numerical simulations of the torque on the bulk plasma due to fast ICRF accelerated ions. The obtained torque is used in a transport equation for the toroidal momentum density to estimate the effect on the thermal bulk plasma rotation profile.

The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and International Space Station. For all these experiments it is important to avoid the strong influence of gravity, exerting an external stress on the system. Whereas PK-3 and PK-3 Plus experiments are using a planar rf capacitive discharge, PK-4 studies complex plasmas in a long cylindrical chamber with a combined dc/rf discharge. Such a configuration of the chamber will provide a particular advantage for investigation of different dynamical phenomena in complex plasmas such as sheared laminar flow of a highly nonideal dusty liquid and its transition to the turbulent regime, nozzle flow, boundary layers and instabilities, shock waves (solitons) formation and propagation, dust particle lane formation, and space dust grain separation according to their size.

The tearing mode (TM) locking and unlocking process due to an external resonant magnetic perturbation (RMP) is experimentally studied in EXTRAP T2R. The RMP produces a reduction of the natural TM velocity and ultimately the TM locking if a threshold in the RMP amplitude is exceeded. During the braking process, the TM slows down via a mechanism composed of deceleration and acceleration phases. During the acceleration phases, the TM can reach velocities higher than the natural velocity. Once the TM locking occurs, the RMP must be reduced to a small amplitude to obtain the TM unlocking, showing that the unlocking threshold is significantly smaller than the locking threshold and that the process is characterized by hysteresis. Experimental results are in qualitative agreement with a model that describes the locking-unlocking process via the balance of the electromagnetic torque produced by the RMP that acts to brake the TM and the viscous torque that tends to re-establish the unperturbed velocity.

A new method for the estimate of the wall diffusion time of non-axisymmetric fields is developed. The method based on rotating external fields and on the measurement of the wall frequency response is developed and tested in EXTRAP T2R. The method allows the experimental estimate of the wall diffusion time for each Fourier harmonic and the estimate of the wall diffusion toroidal asymmetries. The method intrinsically considers the effects of three-dimensional structures and of the shell gaps. Far from the gaps, experimental results are in good agreement with the diffusion time estimated with a simple cylindrical model that assumes a homogeneous wall. The method is also applied with non-standard configurations of the coil array, in order to mimic tokamak-relevant settings with a partial wall coverage and active coils of large toroidal extent. The comparison with the full coverage results shows good agreement if the effects of the relevant sidebands are considered.

Three dimensionless scans in the normalized Larmor radius rho*, normalized collisionality nu* and normalized plasma pressure beta have been performed in JET with the ITER-like wall (JET-ILW). The normalized energy confinement and the thermal diffusivity exhibit a scaling with rho* consistent with the earlier results obtained in the carbon wall JET (JET-C) and with a gyro-Bohm scaling. In the pedestal, experimental results show that the stability is not dependent on rho*, qualitatively in agreement with the peeling-ballooning (P-B) model. The nu* dimensionless scaling shows that JET-ILW normalized confinement has a stronger dependence on collisionality than JET-C. This leads to a reduction of the difference in the confinement between JET-ILW and JET-C to approximate to 10% at low nu*. The pedestal stability shows an improvement with decreasing nu*. This is ascribed to the increase of the bootstrap current, to the reduction of the pedestal width and to the reduction of the relative shift between pedestal density and temperature position. The beta dimensionless scan shows that, at low collisionality, JET-ILW normalized confinement has no clear dependence with beta, in agreement with part of the earlier scalings. At high collisionality, a reduction of the normalized confinement with increasing beta is observed. This behaviour is driven mainly by the pedestal where the stability is reduced with increasing beta. The P-B analysis shows that the stability reduction with increasing beta at high nu* is due to the destabilizing effect of the increased relative shift.

The confinement properties of a reversed-field pinch ( RFP) device are limited by the high level of magnetic stochasticity produced by the interaction of tearing modes in the plasma core. Away to reduce the core magnetic stochasticity is the application of the pulsed poloidal current drive ( PPCD) technique. By applying the PPCD, tearing modes are stabilized, magnetic stochasticity and turbulence are reduced and hence the confinement is enhanced. In the TPE-RX device, standard RFP plasmas are characterized by magnetic and electrostatic edge turbulence dominated by intermittent phenomena; on the contrary, the improved confinement regime generated by the PPCD is characterized by turbulence with a self-similar nature. We show that the increase in tau(p) is well correlated with the change in the nature of both the magnetic and the electrostatic turbulence. While plasmas with low confinement are dominated by intermittent events and by magnetic and electrostatic activity with a non-self-similar nature, plasmas with high confinement show turbulence with a self-similar character.

The physical mechanisms behind the hysteresis in the tearing mode locking and unlocking to a resonant magnetic perturbation (RMP) are experimentally studied in EXTRAP T2R reversed-field pinch. The experiments show that the electromagnetic and the viscous torque increase with increasing perturbation amplitude until the mode locks to the wall. At the wall-locking, the plasma velocity reduction profile is peaked at the radius where the RMP is resonant. Thereafter, the viscous torque drops due to the relaxation of the velocity in the central plasma. This is the main reason for the hysteresis in the RMP locking and unlocking amplitude. The increased amplitude of the locked tearing mode produces further deepening of the hysteresis. Both experimental results are in qualitative agreement with the model in Fitzpatrick et al (2001 Phys. Plasmas 8 4489)

Alfven waves are electromagnetic perturbations inherent to magnetized plasmas that can be driven unstable by a free energy associated with gradients in the energetic particles' distribution function. The energetic particles with velocities comparable to the Alfven velocity may excite Alfven instabilities via resonant wave-particle energy and momentum exchange. Burning plasmas with large population of fusion born super-Alfvenic alpha particles in magnetically confined fusion devices are prone to excite weakly-damped Alfven eigenmodes (AEs) that, if allowed to grow unabated, can cause a degradation of fusion performance and loss of energetic ions through a secular radial transport. In order to control the fast-ion distribution and associated Alfvenic activity, the fusion community is currently searching for external actuators that can control AEs and energetic ions in the harsh environment of a fusion reactor. Most promising control techniques are based on (i) variable fast-ion sources to modify gradients in the energetic particles' distribution, (ii) localized electron cyclotron resonance heating to affect the fast-ion slowing-down distribution, (iii) localized electron cyclotron current drive to modify the equilibrium magnetic helicity and thus the AE existence criteria and damping mechanisms, and (iv) externally applied 3D perturbative fields to manipulate the fast-ion distribution and thus the wave drive. Advanced simulations help to identify the key physics mechanisms underlying the observed AE mitigation and suppression and thus to develop robust control techniques towards future burning plasmas.

This paper reports the progress made at JET-ILW on integrating the requirements of the reference ITER baseline scenario with normalized confinement factor of 1, at a normalized pressure of 1.8 together with partially detached divertor whilst maintaining these conditions over many energy confinement times. The 2.5 MA high triangularity ELMy H-modes are studied with two different divertor configurations with D-gas injection and nitrogen seeding. The power load reduction with N seeding is reported. The relationship between an increase in energy confinement and pedestal pressure with triangularity is investigated. The operational space of both plasma configurations is studied together with the ELM energy losses and stability of the pedestal of unseeded and seeded plasmas. The achievement of stationary plasma conditions over many energy confinement times is also reported.

New experiments at JET with the ITER-like wall show for the first time that ITER-relevant low field side resonance first harmonic ion cyclotron resonance heating (ICRH) can be used to control sawteeth that have been initially lengthened by fast particles. In contrast to previous (Graves et al 2012 Nat. Commun. 3 624) high field side resonance sawtooth control experiments undertaken at JET, it is found that the sawteeth of L-mode plasmas can be controlled with less accurate alignment between the resonance layer and the sawtooth inversion radius. This advantage, as well as the discovery that sawteeth can be shortened with various antenna phasings, including dipole, indicates that ICRH is a particularly effective and versatile tool that can be used in future fusion machines for controlling sawteeth. Without sawtooth control, neoclassical tearing modes (NTMs) and locked modes were triggered at very low normalised beta. High power H-mode experiments show the extent to which ICRH can be tuned to control sawteeth and NTMs while simultaneously providing effective electron heating with improved flushing of high Z core impurities. Dedicated ICRH simulations using SELFO, SCENIC and EVE, including wide drift orbit effects, explain why sawtooth control is effective with various antenna phasings and show that the sawtooth control mechanism cannot be explained by enhancement of the magnetic shear. Hybrid kinetic-magnetohydrodynamic stability calculations using MISHKA and HAGIS unravel the optimal sawtooth control regimes in these ITER relevant plasma conditions.

A loop voltage is required to sustain the reversed-field pinch (RFP) equilibrium. The configuration is characterized by redistribution of magnetic helicity but with the condition that the total helicity is maintained constant. The magnetic field shell penetration time, tau(s), has a critical role in the stability and performance of the RFP Confinement in the EXTRAP device has been studied with two values of tau(s), first (EXTRAP-T2) with tau(s), of the order of the typical relaxation cycle timescale and then (EXTRAP-T2R) with tau(s), much longer than the relaxation cycle timescale, but still much shorter than the pulse length. Plasma parameters show significant improvements in confinement in EXTRAP-T2R. The typical loop voltage required to sustain comparable electron poloidal beta values is a factor of 3 lower in the EXTRAP-T2R device. The improvement is attributed to reduced magnetic turbulence.

High power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition (IPVD) technique that is particularly promising for reactive sputtering applications. However, there are few issues that have to be resolved before the full potential of this technique can be realized. Here we give an overview of the key experimental findings for the reactive HiPIMS discharge. An increase in the discharge current is commonly observed with increased partial pressure of the reactive gas or decreased repetition pulse frequency. There are somewhat conflicting claims regarding the hysteresis effect in the reactive HiPIMS discharge as some report reduction or elimination of the hysteresis effect while others claim a feedback control is essential. The ion energy distribution of the metal ion and the atomic ion of the reactive gas are similar and extend to very high energies while the ion energy distribution of the working gas and the molecular ion of the reactive gas are similar and are much less energetic.

Plasma structures, here typified by the term 'plasmoids', in the solar wind impacting on the magnetopause, i. e. the boundary between the solar wind and the Earth's magnetosphere, can penetrate this boundary and be injected into the magnetosphere. This can happen either by expulsion of the magnetic field from the structure and subsequent diffusion of the magnetic field into the structure or by the formation of a polarization electric field that lets the plasma structure E x B- drift into the earth's magnetic field. In both cases a collisionless resistivity is required at some stage of the process. While magnetic expulsion requires electromagnetic models for its description, polarization can be modelled electrostatically and both processes can be, and have been, studied in laboratory experiments. We present three-dimensional electrostatic particle-in-cell simulations that reproduce large-amplitude waves, in the lower-hybrid range, that have been observed in laboratory experiments. Lower-hybrid waves have also been seen at the magnetopause of the earth. We consider the implications for spacecraft-based studies of magnetopause penetration, and suggest that the search for penetrating plasma structures should emphasize cases in which the interplanetary magnetic field is oriented northwards, as this configuration is less likely for reconnection. The application of theoretical predictions to the magnetopause environment shows that a plasma structure penetrating via polarization needs to be small, i. e. less than 10-100 km wide for typical parameters, and that wave processes at the magnetopause are needed to create such small structures. A larger structure can penetrate by means of magnetic expulsion.

The migration of impurities in tokamaks has been studied with the help of tracer-injection (C-13 and N-15) experiments in JET and ASDEX Upgrade since 2001. We have identified a common pattern for the migrating particles: scrape-off layer flows drive impurities from the low-field side towards the high-field side of the vessel. Migration is also sensitive to the density and magnetic configuration of the plasma, and strong local variations in the resulting deposition patterns require 3D treatment of the migration process. Moreover, re-erosion of the deposited particles has to be taken into account to properly describe the migration process during steady-state operation of the tokamak.

In JET discharges where lower hybrid heating and current drive (LHCD) is applied early during the current ramp, a region of the plasma with zero current density is formed near the axis. At the boundary of this region the current density is large and B-theta increases rapidly over a small distance. In the central region the safety factor, q, is effectively infinite, but this falls steeply in the boundary region. Outside the boundary region q reaches a minimum, where the magnetic shears equivalent to r/q (dq/dr) becomes zero. The formation of this region of zero current is dependent on both the heating and the current drive effects of the LHCD. When LHCD is switched off the current profile begins to relax towards the resistive peaked current distribution of fully inductive tokamak operation. If LHCD is not used in the current rise then these current profiles are not established. Although the physical mechanism exists to drive the central plasma current below zero, in most cases it appears to be prevented from going negative. At least one MHD mechanism has been identified which could be responsible for this. The presence of the zero central current is closely linked to the periodic relaxation events seen in these discharges. In these discharges, internal transport barriers have been observed with additional heating powers substantially below the values required to obtain barriers in monotonic q profile cases.

The rotation of L-mode plasmas in the JET tokamak heated by waves in the ion cyclotron range of frequencies (ICRF) damped on electrons, is reported. The plasma in the core is found to rotate in the counter-current direction with a high shear and in the outer part of the plasma with an almost constant angular rotation. The core rotation is stronger in magnitude than observed for scenarios with dominating ion cyclotron absorption. Two scenarios are considered: the inverted mode conversion scenarios and heating at the second harmonic He-3 cyclotron resonance in H plasmas. In the latter case, electron absorption of the fast magnetosonic wave by transit time magnetic pumping and electron Landau damping (TTMP/ELD) is the dominating absorption mechanism. Inverted mode conversion is done in (He-3)-H plasmas where the mode converted waves are essentially absorbed by electron Landau damping. Similar rotation profiles are seen when heating at the second harmonic cyclotron frequency of He-3 and with mode conversion at high concentrations of He-3. The magnitude of the counter-rotation is found to decrease with an increasing plasma current. The correlation of the rotation with the electron temperature is better than with coupled power, indicating that for these types of discharges the dominating mechanism for the rotation is related to indirect effects of electron heat transport, rather than to direct effects of ICRF heating. There is no conclusive evidence that mode conversion in itself affects rotation for these discharges.

Recent operation of JET with centrally strongly reversed magnetic shear, produced with the help of lower hybrid current drive, has extended the domain in which internal transport barriers (ITBs) can be formed in JET. Performance is frequently limited by magnetohydrodynamic (MHD) instabilities in these reversed shear regimes. The most severe limit is a pressure driven kink mode which leads to a disruption. This disruptive limit is essentially the same in ITB plasmas with low or strongly reversed shear. Unique to the reversed shear regime is a dominantly n = 1 mode, which has multiple harmonics. This mode is a seemingly common limit to performance, in the highest performance plasmas. Also unique to the reversed shear regime are q > 1 sawteeth events, which can in turn trigger n = 1 post-cursor oscillations. In general, these post-cursor oscillations are benign but do provide valuable information on the q-profile. Other instabilities, including 'snakes' at the outer q = 3 surface, are also observed to limit the performance of reversed magnetic shear ITB regimes.

A new technique has been developed to produce plasmas with improved confinement relative to the H 98,y2 scaling law (ITER Physics Expert Groups on Confinement and Transport and Confinement Modelling and Database ITER Physics Basics Editors and ITER EDA 1999 Nucl. Fusion 39 2175) on the JET tokamak. In the mid-size tokamaks ASDEX upgrade and DIII-D heating during the current formation is used to produce a flat q-profile with a minimum close to 1. On JET this technique leads to q-profiles with similar minimum q but opposite to the other tokamaks not to an improved confinement state. By changing the method utilizing a faster current ramp with temporary higher current than in the flattop (current overshoot) plasmas with improved confinement (H 98,y2=1.35) and good stability (Î² N3) have been produced and extended to many confinement times only limited by technical constraints. The increase in H 98,y2-factor is stronger with more heating power as can be seen in a power scan. The q-profile development during the high power phase in JET is reproduced by current diffusion calculated by TRANSP and CRONOS. Therefore the modifications produced by the current overshoot disappear quickly from the edge but the confinement improvement lasts longer, in some cases up to the end of the heating phase.

Recent studies (Maggi et al 2015 Nucl. Fusion 55 113031; Maggi et al 2017 Nucl. Fusion 57 116012) have shown that on JET with the Be/W ITER-like wall (JET-ILW) in high beta discharges with high D-2 gas rates, the inter-ELM temperature pedestal growth is saturated half way through the ELM cycle, leading to plasmas with reduced confinement, and that the linear MHD stability of these pedestals is inconsistent with the peeling-ballooning paradigm (Snyder et al 2002 Phys. Plasmas 9 2037-43; Wilson et al 2002 Phys. Plasmas 9 1277-86). In this paper, the inter-ELM evolution of the edge current density is investigated in a wide range of type. I ELMy H-modes on JET-ILW. It is found that in discharges at a low gas rate, the peak edge bootstrap current continuously increases until the ELM crashes, while it saturates during the ELM cycle at high gas rates. The effect of current diffusion on the build-up of the edge current inter-ELM is assessed by simulating the Ohmic current contribution with the JETTO transport code. The simulations indicate that current diffusion contributes little to the time evolution of the total edge current in the second half of the ELM cycle and the total current is dominated by the bootstrap current. Therefore, current diffusion does not explain why JET-ILW type. I ELMy pedestals at a high gas rate and high beta(N) are found to be stable to peeling-ballooning modes.

The modelling of Alfvenic instabilities is discussed from the point of view of mode-conversion, showing how the development of the theory affects the predictions as the limitations of the models are gradually relaxed. Conventional tokamak plasmas are relatively well understood and are used for the case of a kinetic Alfven eigenmode (AE) to assess the resonant wave-particle interactions along the magnetic field. The large safety factor in the core of deeply reversed shear plasmas and the low magnetic field of spherical tokamaks, however, bring the AEs down into the drift-frequency range; modifications of the spectrum through toroidal mode-conversion then creates a new class of drift-kinetic AEs that could affect the fast particle confinement. Experiments have been carried out to verify these predictions in JET. They confirm the presence of weakly damped modes, which do not follow the usual AEs scaling.

Ray optics is used to model the propagation of short electromagnetic plasma waves in toroidal geometry. The new RAYCON code evolves each ray independently in phase space, together with its amplitude, phase and focusing tensor to describe the transport of power along the ray. Particular emphasis is laid on caustics and mode conversion layers, where a linear phenomenon splits a single incoming ray into two. The complete mode conversion algorithm is described and tested for the first time, using the two space dimensions that are relevant in a tokamak. Applications are shown, using a cold plasma model to account for mode conversion at the ion-hybrid resonance in the Joint European Torus.